Fetal monitoring (i.e., monitoring of the fetal condition during gestation and at birth) usually comprises monitoring of the uterus activity (toco) and of the fetal beat-to-beat heart rate (FHR). The fetal heart rate is the more important of these parameters, as it provides an indication of whether the fetus is sufficiently supplied with oxygen. Of course, both parameters may also be used for further diagnostic statements. In particular, the relation between fetal heart rate and labor is used to evaluate the fetal condition. The present invention relates to the calculation of the fetal heart rate. This heart rate is preferably calculated from beat to beat; i.e., the heart rate is calculated after each beat by means of the time interval to the preceding beat. The beat-to-beat heart rate is most meaningful for diagnostic purposes; however it is also possible to calculate the fetal heart rate over longer time intervals or to average the beat-to-beat heart rate.
To obtain a signal indicative of the fetal heart rate, a so-called fetal scalp electrode may be applied to the fetal skin. These electrodes are usually spiral electrodes which are screwed into the fetal epidermis. The electrodes provide very accurate measurements due to the excellent signal quality. Unfortunately, this so-called internal or direct measurement may only be used after rupture of the membranes. Prior to that point in time (in particular, during gestation), indirect methods must be used. The indirect measurements are performed abdominally, e.g., by listening to the fetal heart sound or by measuring the Doppler shift of an ultrasound signal reflected by the moving fetal heart.
The ultrasound technique is the most common one. According to this technique, an ultrasound transducer is placed externally on the pregnant woman's abdomen. Its orientation is selected such that the emitted ultrasound waves reflect off the fetal heart. The reflected ultrasound wave is received either by the same or by another ultrasound transducer. The Doppler shift of the reflected ultrasound wave is directly related to the speed of the moving parts of the heart, e.g., the heart valves and the heart walls.
To extract the Doppler component from the received ultrasound signal, the received ultrasound signal is demodulated. Further processing depends on the specific application. For example, the Doppler signal may be fed to an audio amplifier, thus giving an acoustic indication of the heart beat; however, in order to generate a digital representation of the heart rate, more sophisticated techniques must be used. One such sophisticated technique which is commonly used in fetal monitors is the autocorrelation technique. According to this technique, the Doppler signal or the envelope of the Doppler signal is correlated with itself, thus providing significant peaks in time intervals which correspond to periodic components of the Doppler signal that are due to the fetal heart beat. Such techniques are necessary as the received ultrasound signals contain noise originating from various physiological sources, such as the maternal aorta, movement of the fetus as a whole, and the like.
In prior art devices, a peak trigger device or an equivalent circuit detects the peaks in the autocorrelation function. The beat-to-beat heart rate, computed as the inverse of the time interval between two successive heart beats, is then available for further processing, display and/or recordation.
Even the highly sophisticated autocorrelation technique is not always reliable. In particular, the autocorrelation function may contain several maxima, so that it becomes difficult to distinguish between significant maxima relating to a heart beat and secondary maxima. Moreover, the autocorrelation function may be distorted so that it is impossible to identify peaks at all. In these cases the calculated fetal heart rate may be inaccurate, or it may even be impossible to obtain a heart rate signal at all.
In the prior art circuit depicted in FIG. 1, a 1 MHz signal is fed via line 1 to a transmission/reception control circuit 2. The signal is then fed via a high-frequency amplifier 3 to a transducer 4. This transducer comprises piezo-electric crystals which are capable of transmitting and receiving ultrasound waves. In clinical applications, the ultrasound transducer 4 is placed on the maternal abdomen such that the ultrasound waves are emitted in the general direction of the fetal heart. The ultrasound waves reflected by the fetal heart and other fetal tissue are then received by ultrasound transducer 4 and fed to a second high-frequency amplifier 5. The ultrasound waves are irradiated into the maternal and fetal tissue as burst waves, i.e., "packages" of high-frequency (1 MHz) waves. The repetition rate of the bursts is 3.2 kHz. This is illustrated in FIG. 2a. The signal depicted in FIG. 2a is the signal which is fed from transmission/reception control circuit 2 to amplifier 3 via line 6.
The emitted ultrasound signal consists of bursts 7, 8, etc. The bursts are emitted in time intervals of 312.5 .mu.sec, which corresponds to a repetition rate of 3.2 kHz. The frequency of the waves within the bursts is 1 MHz, i.e., the time interval between their respective amplitudes is 1 .mu.sec, as illustrated in FIG. 2a.
The signal generated by high-frequency amplifier 5 is then fed via line 9 to a demodulator circuit 10 ("DM"). Demodulator circuit 10 also receives a control signal from transmission/reception control circuit 2 on line 11. The control signal on line 11 is also depicted in FIG. 2b. Burst 12 of this signal is shifted in time with respect to burst 7 in FIG. 2a. The time shift is labelled as .DELTA.t in FIG. 2b. This time shift takes into account the propagation delay between the onset of the emitted ultrasound burst and the time when the reflected ultrasound wave is received. .DELTA.t may be varied in accordance with the distance between the ultrasound transducer 4 and the fetal heart.
Demodulator 10 produces a signal which is indicative of the Doppler shift of the received ultrasound signal with respect to the transmitted signal. The Doppler shift is caused by the moving parts of the fetal heart, in particular the heart walls and the heart valves.
The Doppler signal produced by demodulator 10 is fed, via line 13, to a bandpass filter with a pass band between 100 and 500 Hz to remove unwanted components in the Doppler signal. The signal is then fed, via line 15, to an envelope demodulator 16 ("EDM"). The envelope demodulator generates the envelope of the peaks of the Doppler signal, as the signal of interest is primarily the intensity of the Doppler signal.
Line 17 connects the output of envelope demodulator 16 with a gain control circuit 18. After having passed the gain control circuit, the signal is fed via line 19 to an autocorrelation circuit 20. Autocorrelator circuit 20 correlates the signal received on line 19 with itself. A major feature of the autocorrelation function is that periodic components in the incoming signal are amplified, whereas non-periodic or stochastic signals are widely suppressed; therefore, autocorrelation is ideally suited to amplify the periodic components, in contrast to non-periodic signals, e.g., signals caused by fetal movement, maternal tissue or the like.
The basic equation for calculating the autocorrelation function is ##EQU1## wherein r(t) is the incoming (autocorrelated) signal. FIG. 3 depicts a typical autocorrelation function A.sub.r(t) over time. It will be noted that the amplitude of the autocorrelation function is at its maximum A.sub.0 at t=0. Further maximums occur at t=.tau., t=2.tau., t=3.tau., and so on. .tau. in this notation represents the time between one significant peak of the autocorrelation function and the next significant peak. The significant peaks in the autocorrelation function relate to fetal heart beats and have an approximately equidistant separation between successive peaks. Their amplitude is labelled as A.sub..tau. in FIG. 3. It should be noted that the above equation describes an ideal autocorrelator. It will be understood that a real autocorrelator approximates this function, e.g., by taking the sum instead of the integral, and by finite integration limits instead of the infinite limits given above.
The autocorrelation function generated by autocorrelator 20 is used to calculate the fetal heart rate. This is done by evaluation circuit 21 to which the output of autocorrelator 20 is fed via line 22. In fetal monitoring applications, the beat-to-beat heart rate has the highest diagnostic value. It is calculated as the inverse of the time interval between two adjacent peaks in the autocorrelogram. It should be noted, however, that there are other ways to calculate the heart rate, e.g., computing it over a longer time interval, averaging the time intervals or averaging the heart rate.
The generated heart rate ("HR") is fed via line 23 to further circuitry, e.g., a display, a recorder, a central station or the like.
Evaluation circuit 21 also provides a liability coefficient "CONF" via line 24. The reliability coefficient indicates the quality of the autocorrelation function and therefore the quality of the received ultrasound wave. This is important for the physician or the nurse, as a poor-quality signal may result in an incorrect heart rate. Such may happen, e.g., if the ultrasound beam is not sufficiently focussed on the fetal heart. A low reliability coefficient is therefore an indication that the ultrasound transducer should be readjusted. One way of indicating the reliability or quality of the signal is to use lamps or light-emitting diodes of different colors (e.g. red, yellow and green).
The reliability coefficient may be calculated in different ways. Referring to FIG. 3, a preferred solution is to calculate the ratio between the amplitude A.sub..tau. of a peak relating to a heart beat and the amplitude A.sub.0 of the autocorrelation function when the signal is correlated with itself. The coefficient ##EQU2## is a quite good estimation of the quality of the signal. The reliability coefficient may also be calculated or determined in a different manner, e.g., by taking the absolute amplitude A.sub..tau., by determining the area under a significant peak (by integration), or the like.
The inventors of the present invention have found that the deficiencies of the prior art autocorrelation technique, and therefore the deficiencies of the fetal heart rate obtained with prior art monitors using this technique, are primarily caused by two effects: First, the Doppler shift of the reflected ultrasound signal originates from various moving boundary surfaces in the fetal heart. The corresponding components of the Doppler signal change their frequency as well as their amplitude independent of each other if the angle of the incident ultrasound wave is only slightly changed. This results in a very complex Doppler signal (or envelope signal) which changes over time. An autocorrelation function based upon this signal contains not only the main maxima (or significant peaks) corresponding to heart beats, but also various secondary peaks varying over time. These secondary maxima or peaks affect the evaluation of the autocorrelogram and therefore also affect the determination of the beat-to-beat heart rate. The second problem originates from the fact that not all components of the received ultrasound signal originate from the fetal heart. Such components may be superimposed on the reflections of the fetal heart and may even cover them. Determination of the fetal heart rate by means of an autocorrelation technique may deliver inaccurate results if such happens.
It is therefore a major objective of the present invention to provide a method and apparatus which generate more reliable heart rate signals, particularly, more reliable fetal heart rate signals.